The present invention relates to the coupling of light between a pixel comprising a light-sensor, and/or a light-emitter, and/or a light-valve, and other optical elements such as wavelength filters, polarization filters, beam splitters, etc. Such optical elements can be implemented with a varied set of technologies, including refractive optics, diffractive optics, 1D, 2D or 3D photonic crystals (or photonic band-gap materials), including purely dielectric structures, and/or metallodielectric structures, and/or metal structures with the possibility of using surface plasmon-polariton (SPP) effects.
Standard image sensors have microlens arrays, also referred to as microlenticular arrays or lenslet arrays, to focus the incoming light rays into the photo-diode region of each pixel, which in certain types of CCDs and in all types of CMOS Image Sensors (CIS) is only a fraction of the size of the total pixel. The ratio of the area of the light-sensing region over the entire pixel area is called the “Fill Factor”. Therefore microlenses increase the optical fill factor over the pixel layout fill factor.
Typically, microlenses are fabricated on top of the Color Filter Array (CFA), which itself is fabricated on top of the passivation layer deposited over the last metal level of the image sensor integrated circuit.
This architecture for color filtering and maximizing the optical fill factor of imaging systems establishes a framework within which there has been incremental progress in optimizing each of these factors and their overall integration.
However, CIS made with CMOS technology of a particular generation, for example 0.13 μm, have not been able to use all the metal levels available for version of the process used to fabricate purely digital CMOS integrated circuits. This is because increasing the number of metal levels in the image sensor integrated circuit, inevitably increases the distance between the top of the passivation layer (on which the CFA and microlenses are made) and the photo-diode regions in the pixel matrix. As this distance increases, it becomes more likely that light focused by the microlens pertaining to a certain pixel, will actually be absorbed by the photo-diode of an another pixel. This is especially true for light rays that impinge on the microlens at high angles (angles far from the perpendicular to the substrate). This is also known as the “microlens problem”. FIGS. 1A, 1B and 1C (Prior Art) show a typical cross section view of an image sensor, depicting the “microlens problem”, especially visible in FIGS. 1B and 1C, in which a given microlens directs light to the wrong photo-diode.
For black & white image sensors, this problem results in light intensity cross-talk, and for color image sensors using CFAs, it also results in color cross-talk. Because of the severity of this problem, the number of metal layers used for on-chip interconnects has been generally kept to 4 or less.
Furthermore, after passing through the microlenses, light rays proceed to cross the color filters, the passivation layer, and all the interfaces between layers of different dielectric materials until they finally reach the surface of the light-sensing regions. All these layers have interfaces from which light is reflected back, and therefore never reaches the photo-sensing regions. FIG. 2 (Prior Art) shows reflection of light at the interfaces between different materials in the metallization stack. This problem is especially critical for copper metallization, which typically has one or more barrier layers (such as silicon nitride) at each metal level, resulting in a large number of interfaces between materials with significantly different indices of refraction, causing the reflection of an important fraction of the impinging light rays.
For CIS it would be highly advantageous to increase the number of metal levels possible to fabricate, because it would improve the performance and functionality of on-chip circuitry, which is one of the most attractive advantages of CIS over CCDs.
Conventional microlenses are typically made with soft materials, such as particular types of photo-resists, that are transparent to, and refract, light in the desired range of wavelengths (typically the visible range).
Microlenses are the last structures to be made during the wafer level fabrication of image sensors, because, if for no other reason, they are very sensitive to, and can be easily degraded by, the typical temperatures used in chemical vapor deposition (CVD), which is the preferred method to deposit films such as, for example, silicon dioxide and silicon nitride, or even diamond.
Also, one reason for making microlenses with such materials, is due to the fact that they are made on top of color filters, which are themselves also very sensitive to processing temperature. Were it not for the color filters, microlenses could be made with more rugged materials, such as diamond for example.
It should also be noted that in conventional image sensors, the plane of the microlenses is placed at the focal plane of the system lens. In order to avoid strong reflections from the top surface of the microlenses, the index of refraction of said microlenses should not be much larger than that of air, which, typically, is the only medium between the system lens and the microlenses.
As to the color filtering architecture used in standard imaging systems, it consists in the fabrication of a color filter array having each color filter aligned with a different pixel. The best known CFA is the Bayer pattern, in which there are two filters for green, one filter for red and one filter for blue. The filters consist of a thin film of an organic or inorganic material that allows the transmission of photons in the wavelength range corresponding to its color. However, the spectral purity of these filters is fairly poor in the sense that the transmission curve has a slow decrease towards zero for the wavelengths outside the desired interval, thereby significantly overlapping wavelengths that should only be transmitted by filters of other colors. The spectral purity of the filters can usually be increased by forming thicker films of the material used for color filtering, but that results in less light transmitted for the desired wavelengths. Therefore, for each color filter there is a tradeoff between spectral purity and transparency. Regardless of spectral purity, conventional color filtering always causes some attenuation in the wavelengths of interest.
In addition, the typical materials used for color filtering are intolerant of temperatures necessary to perform further processing using standard microstructure fabrication methods such as CVD for example, thereby placing severe restrictions on what other materials and structures can be fabricated after the formation of color filters. For example, from several points of view there would be many advantages to fabricating microlenses with materials such as diamond, due to its large index of refraction across a large range of wavelengths from the UV to the LWIR. However the temperatures and process steps necessary to deposit and pattern diamond cannot be tolerated by the typical materials used for color filters.
The present invention addresses the problems discussed above and provides an alternative structure to maximizing the optical fill factor and performing color filtering, that enables new possibilities for imaging systems. The new structure provides solutions, and the methods for their fabrication, to the problem of coupling light between the area corresponding to a pixel at the focal plane, located above the metallization stack, and the photo-diode region of the corresponding pixel, as well as to the problem of loss of light by the reflections at the interfaces between different materials in the metallization stack.
The structure according to the invention can be used in conjunction with, or substitute microlenses, depending on certain aspects of the implementation. In addition, the present invention may also be used in conjunction with, or it may substitute, conventional color filter arrays (CFAs), depending on certain aspects of the implementation. It is also possible to have an implementation in which both, the conventional microlens array and the conventional color filter array, can be substituted by the present invention.
In a scenario in which the new structure replaces conventional microlenses and color filters, after their fabrication, further processing becomes possible, including the formation of a region of high index of refraction, along with anti-reflection structures, at the focal plane produced by the system lens. Consequently, the image circle produced by the system lens has its lateral dimensions reduced, with respect to air, by a numerical factor given by the ratio of the indices of refraction of said region with high index of refraction over that of air. In this case, the lateral dimensions of the pixels in the imaging matrix, and the dimensions of the matrix itself, can also be scaled by the same numerical factor. It should be noted that this will not change the amount of light received by each pixel.